The present invention relates to a testing bed, and also to apparatus including such a testing bed, for testing single-layer or multi-layer PCBs (printed circuit boards), MCMs (multi-chip modules), and other articles having electrically-conductive nets (e.g., electrically-conductive pathways) on a surface thereof. The invention is particularly useful in apparatus for electrically testing for discontinuities in nets, and/or for shorts between nets, of PCBs, hybrid circuits, MCMs and other like articles, and therefore the invention is described below particularly with respect to such testing apparatus.
Electrical testing apparatus of the foregoing type commonly includes a plurality of probe elements for contact with pads (test or functional points) of the nets for measuring the resistance of the net between selected pads to indicate a discontinuity in the net, or for measuring capacitance between selected nets to indicate a short between nets.
One known type of testing apparatus for this purpose is based on "flying" testing beds. Such apparatus includes a line of spring-biased pins that are pressed into contact with the PCB, MCM, or other article to be tested, as the pins are moved with respect to the article, or vice versa, to create contact with each net to be tested in its turn. This type of apparatus, however, is very slow and therefore not economical for testing most types of PCBs and similar articles being produced at high volume.
A more popular apparatus for testing PCBs and similar articles in high volume production is based on the use of a "bed of nails" testing bed fixture. Such a fixture includes a large number of spring-biased pins arranged in the form of a matrix or grid pressed into contact with the tested article. However, such a testing bed fixture generally must be custom-made for each type of article to be tested, and its expense increases drastically with an increase in resolution because of the mechanical connections required between the pins and the pads. As a practical matter, the testing bed resolution using such a fixture is limited to about 20 mils, and the test accuracy decreases with increased resolution. This is not sufficient for many applications at the present time, and will become even less sufficient in the future with the increasing density of nets in PCBs and SMT (surface mounting technology) strips, and very dense MCMs. Moreover, the testing time in existing machines using a "bed of nails" testing bed fixture is relatively long, and will become longer as the pad density increases, because of the special arrangements required for providing appropriate interfaces between the testing bed fixture and the output circuitry.
Another disadvantage in using both the "bed of nails" and the "flying" testing beds is that the spring-biased pins in such testing beds are liable to damage the surface of the PCB or other article to be tested, in particular, thin film products such as MCMs.
It has also been proposed to use testing apparatus which includes selectively illuminated photoconductive gates. For example, U.S. Pat. No. 4,709,141 discloses such apparatus for testing arrays of infrared detectors which are individually addressed; but such apparatus would not be usable for testing a plurality of pads simultaneously, or for testing the conductivity of two circuit elements on a straight line. U.S. Pat. No. 5,177,437 discloses a complicated construction which includes lower and upper matrices to be located on opposite sides of the board to be tested.